1. Field of the Invention
The present invention relates to a liquid crystal display apparatus, and especially to a twisted nematic liquid crystal display apparatus having a wide view angle.
2. Description of Related Art
Ever since the liquid crystal display was invented, it has fulfilled the consumer's expectations and demands on thin and light designs for a display with its light weight, thinness, and small size. Moreover, compared with the cathode ray tube (CRT) display, the liquid crystal display has excellent features such as low power consumption and low radiation, which make the liquid crystal display one of the most important electronic products in today's human life.
However, the liquid crystal display is limited by its principle of light emission. There remain some problems that urgently need mitigating. For instance, the range of the view angle in the liquid crystal display is too narrow. When people view the liquid crystal display apparatus with a larger horizontal or vertical view angle, the display quality is then lowered. That is, the contrast ratio, the color saturation and the luminance of the liquid crystal display apparatus are not as good as during a right view.
Furthermore, the definition of a view angle is the angle when the contrast of a display is smaller than 10. The liquid crystal display is generally indicated in eight-digit gray scales, i.e. 256 different gray scales, with the zero gray scale being the full black state, and the 255 gray scale being the full white state. Normally, a liquid crystal display in a high level state is brighter than one in a low level state. If the luminance of a liquid crystal display in a low level state is instead brighter than the luminance of one in a high level state, it is called the gray scale inversion. This phenomenon usually happens when people view the liquid crystal display with a too wide view angle.
Generally, the twisted nematic-LCD (TN-LCD) has the smallest view angle range because the twist angle of a liquid crystal atom is smaller. The horizontal view angle is only around 45 degrees on both the right and the left directions, which still leaves much room for improvement. Therefore, in order to solve the problem of the view angle, a compensation film is used to widen the view angle of the twisted nematic LCD.
FIG. 1 is a schematic view showing the structure of a conventional twisted nematic LCD apparatus. Referring to FIG. 1, this twisted nematic LCD apparatus 100a generally includes a thin film transistor (TFT) array substrate 110, a color filter (CF) substrate 120, a first polarizer 130, a second polarizer 140, a twisted nematic liquid crystal layer 150. The TFT array substrate 110 is disposed on the first polarizer 130, and the color filter substrate 120 is disposed over the TFT array substrate 110. The second polarizer 140 is disposed on the color filter substrate 120, and the twisted nematic liquid crystal layer 150 is disposed between the TFT array substrate 110 and the color filter substrate 120.
FIG. 2 is the measurement results of the horizontal view angles of the conventional twisted nematic LCD apparatus 100a. Referring to FIG. 2, the horizontal axis is the horizontal view angle, the vertical axis is the transmittance, and the vertical axis also represents luminance. In addition, different curves show the results of the conventional twisted nematic liquid crystal display apparatus 100a under different operating voltages.
It can be seen from FIG. 2 that when the conventional twisted nematic LCD apparatus 100a is at zero volt, since the twisted nematic LCD apparatus 100a is in a high gray scale display state, the LCD has the highest transmittance at this moment. It indicates the conventional twisted nematic LCD apparatus 100a has the highest luminance. Afterwards as the operating voltage increases, the luminance displayed on the conventional twisted nematic LCD apparatus 100a gradually decreases. It is noted that around 45 degrees and −45 degrees on the horizontal axis as shown in FIG. 2, some curves cross one another at this section, which indicates the transmittance at high operating voltages is higher than the transmittance at low operating voltages. In other words, the luminance in low gray scales is brighter than the luminance in high gray scales; this phenomenon is the foregoing gray scale inversion. In view of the above, the view angle range of the conventional twisted nematic liquid crystal display apparatus 100a is only 45 degrees on the right and the left along the horizontal direction.
FIG. 3 is a schematic view showing the structure of another conventional twisted nematic liquid crystal display apparatus. Referring to FIG. 3, FIG. 3 is similar to FIG. 1. The difference between the two is that in order to widen the view angle, a conventional twisted nematic liquid crystal display apparatus 100b further includes a first compensation film 160 and a second compensation film 170. The first compensation film 160 is disposed between the first polarizer 130 and the TFT array substrate 110, and the second compensation film 170 is disposed between the second polarizer 140 and the color filter substrate 120. However, two extra compensation films 160 and 170 have to be added into the structure. As a result, no only are the cost and the overall thickness of the display increased, the additional elements also influence to a certain extent the reliability, the production yield and the life time of the liquid crystal display.